As well known among people skilled in the art, a composite synthetic resin material including an outer synthetic resin layer, and at least one inner synthetic resin layer wrapped in the outer synthetic resin layer is often used as a synthetic resin material for forming a preforming product (generally called a preform) to be blow molded into a beverage container, or for forming a container closure. Usually, a synthetic resin excellent in mechanical properties and hygienic properties is selected as the outer synthetic resin, while a synthetic resin excellent in gas barrier properties is selected as the inner synthetic resin.
Japanese Published Unexamined Patent Application No. Hei 1-195016 discloses a composite synthetic resin material forming apparatus comprising an outermost channel having an outermost outlet, an outer channel having an outer outlet opening into the outermost channel, an inner channel having an inner outlet opening into the outer channel, an outermost synthetic resin supply means for flowing an outermost synthetic resin through the outermost channel, an outer synthetic resin supply means for flowing an outer synthetic resin through the outer channel, and an inner synthetic resin supply means for flowing an inner synthetic resin through the inner channel. The inner outlet has an opening and closing means annexed thereto for selectively opening and closing the inner outlet. In accordance with the opening and closing of the inner outlet by the opening and closing means, the inner synthetic resin is intermittently flowed from the inner channel into the outermost channel via the outer channel. This inner synthetic resin is wrapped in the outermost synthetic resin directly flowed into the outermost channel, and the outer synthetic resin (the outer synthetic resin and the outermost synthetic resin may be the same synthetic resin) flowed into the outermost channel via the outer channel. As a result, a composite synthetic resin material is flowed out of the outermost outlet.
Japanese Published Unexamined Patent Application No. Hei 6-15715 also discloses a composite synthetic resin material forming apparatus comprising an outermost channel having an outermost outlet, an outer channel having an outer outlet opening into the outermost channel, an inner channel having an inner outlet opening into the outer channel, an outermost synthetic resin supply means for flowing an outermost synthetic resin through the outermost channel, an outer synthetic resin supply means for flowing an outer synthetic resin through the outer channel, and an inner synthetic resin supply means for flowing an inner synthetic resin through the inner channel. The outermost synthetic resin supply means is continuously operated to continuously flow the outermost synthetic resin directly into the outermost channel. The inner synthetic resin supply means is intermittently operated to intermittently flow the inner synthetic resin into the outermost channel via the outer channel. The outer synthetic resin supply means is intermittently operated a required time behind the intermittent operation of the inner synthetic resin supply means to intermittently flow the outer synthetic resin (the outer synthetic resin and the outermost synthetic resin may be the same synthetic resin) into the outer channel.
However, the composite synthetic resin material forming apparatus disclosed in the aforementioned Japanese Published Unexamined Patent Application No. Hei 1-195016 has the following problems to be solved: Particularly when the composite synthetic resin material is to be flowed out of the outermost outlet at a relatively high speed, the inner synthetic resin, which has been wrapped in the outermost synthetic resin and the outer synthetic resin, leaves a relatively long, thin trail in the flowing direction. Thus, the length, in the flowing direction, of a region where the inner synthetic resin is not present becomes markedly short. This makes it necessary to cut the composite synthetic resin in a markedly restricted region in the flowing direction of the composite synthetic resin, when cutting the composite synthetic resin, which has been discharged from the outermost outlet, in a direction perpendicular to the flowing direction to cut off the composite synthetic resin from the composite synthetic resin forming apparatus. Owing to a slight error in the cutting, therefore, it is highly likely that a defective composite synthetic resin material having the inner synthetic resin exposed at the cut surface will be formed.
In the composite synthetic resin material forming apparatus disclosed in the above-described Japanese Published Unexamined Patent Application No. Hei 6-15715, the outer synthetic resin is intermittently caused to inflow a predetermined time behind the intermittent inflow of the inner synthetic resin to pressurize the inner synthetic resin from the upstream side in the flowing direction, thereby shortening the length, in the flowing direction, of the inner synthetic resin so that the above problem with the composite synthetic resin material forming apparatus disclosed in the aforementioned Japanese Published Unexamined Patent Application No. Hei 1-195016 can be solved. However, the composite synthetic resin material forming apparatus disclosed in the above-described Japanese Published Unexamined Patent Application No. Hei 6-15715 is still not fully satisfactory. Particularly when the composite synthetic resin is flowed at a considerably high velocity, the phenomenon of the inner synthetic resin leaving a long trail is not sufficiently suppressed, partly because no opening and closing means is annexed to the inner outlet. Thus, a relatively long, thin trail of the inner synthetic resin tends to be generated.